Making Sense of Abrupt Climate Change, or What Would Yoda Do?

Author:  Hsu Jeremy
Institution:  History of Science and Technology
Date:  October 2004

When most people think of climate change, one word comes to mind – slow. But, abrupt climate change made headlines in February when the Pentagon released a report commissioned by senior official Andrew Marshall, director of the Office of Net Assessment. The report detailed a hypothetical situation in which abrupt climate change wreaks havoc on nations worldwide.

The publicity-shy, 82-year old Marshall, known as the Pentagon's "Yoda," intended the report to be a what-if scenario solely for policy planning purposes. Instead, several newspapers took the Pentagon report to be a literal prophecy of doom. The London Observer was first to announce, "Now the Pentagon tells Bush: climate change will destroy us." It was soon followed by other news stories with equally alarming tones.

The incident was more than a case of inaccurate journalism. The media coverage of the Pentagon report reflects a broader political desire for certainty from a science that is highly uncertain. Abrupt climate change is neither certain nor likely in the near future, but its consequences are potentially grim enough that scientists and policy experts are taking it very seriously.

Abrupt Climate Change: What Is It?

The Pentagon scenario was inspired by a 2002 National Research Council (NRC) report titled "Abrupt Climate Change: Inevitable Surprises," which includes the current scientific understanding of abrupt climate change. Scientists previously thought that climate changes occurred gradually over tens of thousands of years. But, recent geological evidence shows climate changes can occur within mere decades.

Many scientists link abrupt climate change to disruption of the ocean conveyor belt, specifically the Thermohaline Circulation (THC) of the North Atlantic. The ocean conveyor is part of the climate system that carries heat into the northern atmosphere. It works because of the Atlantic Ocean's high salinity, or saltiness, which makes cold water denser. As the water flowing northward cools, it sinks down into the deep ocean and is replaced by warm water flowing at the surface. The higher the salinity of the water, the faster the ocean conveyor's warm water flows north to release heat into the northern atmosphere.

Abrupt climate change can occur when melting glaciers or ice sheets release a massive amount of fresh water into the ocean, making the sea water less salty. The lower salinity means that the cold water does not sink as rapidly or at all, and the flow of warm water slows. As a result, the ocean conveyor becomes slower or may even collapse, and less heat is released into the northern latitudes. This can result in colder annual average temperatures for certain northern regions like Europe.

"If you believe the [climate] models, you can't stop the conveyors without a major warming of the planet," says Wallace Broecker, a geochemist at Columbia University's Lamont-Doherty Earth Observatory and the first scientist to link ocean conveyor disruption to abrupt climate change. He believes that the ocean conveyor only collapses when the planet is warmed by 7 to 11 degrees Fahrenheit.

Broecker's model suggests that since the Earth is currently getting warmer, abrupt climate change may be a greater possibility. But, while the vast majority of scientists agree that the Earth is warming, some scientists question if disruption of the ocean conveyor is the primary cause behind abrupt climate change.

"One of the alternative possibilities is changes in circulation respond to changes in tropical oceans.the stratosphere [also] has more effect on surface conditions than previously thought," explains John Wallace, an atmospheric scientist at the University of Washington in Seattle and an author of the NRC report. He notes that the ocean conveyor theory has been around longer, and therefore has been discussed much more in the popular press.

All scientists agree that abrupt climate change is "highly uncertain at this point," as Wallace puts it. Broecker and Wallace also think that the possibility of abrupt climate change is a strong reason for preventing global warming.

The most recent abrupt climate change event occurred 8,200 years ago and caused temperature drops of up to five degrees Fahrenheit in a few decades. Severe winters in Europe and other regions resulted in advancing glaciers, frozen rivers, and damage to agriculture. This century-long event became the model for the Pentagon report's scenario.

Making Decisions in Uncertainty: Science and Policy

Planning around risk and uncertainty is common in the policy world. Just as people have insurance to protect against risks to their cars and houses, policy planners want to be prepared for risks to the United States and even the world. Far from being a weather forecast of the future or a scientific prediction, Marshall's Pentagon report merely considers preparations for a worst-case scenario of abrupt climate change.

When Andrew Marshall read the NRC report on abrupt climate change, he contracted planners Peter Schwartz and Doug Randall to examine the potential social and economic impacts on nations. Schwartz and Randall consulted with leading climate scientists to come up with a plausible, if not most likely, abrupt climate change scenario.

The Pentagon report imagines a future where droughts cause famine, flooding threatens coastal cities, and resources ranging from water to energy become scarce. America and Australia become fortresses as nations struggle with displaced refugee populations and begin warring over access to limited resources.

"This report suggests that.the risk of abrupt climate change.should be elevated beyond a scientific debate to a U.S. national security concern," Schwartz and Randall conclude.

The Pentagon report recommends improving predictive climate models, but it also recommends identifying "no-regrets" strategies that work regardless of future climate changes. These strategies include taking steps to ensure that storm damage to coastal cities is minimized, or ensuring that enough food is stored to survive long periods of drought. The advantage of "no-regrets" strategies over predictive climate models is that they can be enacted despite being uncertain about when abrupt climate change may occur.

"Any federal agency should be thinking about preparing for abrupt climate change," says Roger Pielke, policy expert from the University of Colorado, Boulder and another author of the NRC report. He compares abrupt climate change to an asteroid impacting the earth, a "low probability, high consequence event" for which policymakers should prepare.

Daniel Sarewitz, director of the Columbia University Center for Science, Policy and Outcomes in Washington, D.C., makes a similar analogy by comparing abrupt climate change to an earthquake rather than a terrorist attack. For earthquakes, policy experts focus on preparation; for terrorism, they focus on prevention. Like an earthquake, abrupt climate change is highly unpredictable and cannot be controlled. Therefore preparation is critical since prevention may not be possible.

"The issue is less one of can we predict what is going to happen and then do something to prevent it, as opposed to minimizing risk and maximizing preparation," says Sarewitz. However, he adds that more knowledge about abrupt climate change would be helpful.

Both policy experts also stress that abrupt climate change is a related but separate issue from global warming. Pielke observes that public debate on abrupt climate change has become a "justification for addressing global warming" instead of a serious discussion of abrupt climate change itself. He suggests that a climate monitoring system would be useful to detect climate changes, whether abrupt or gradual, human-caused or natural.

After the Pentagon report was released, author Peter Schwartz said in an NPR interview that media distortions and hype had made the report a "taboo topic" for serious discussion. But, though the report has been buried for now at the Pentagon, discussions of abrupt climate change continue in science and policy circles.

The United States and the world will probably face the issue of abrupt climate change again. The only question is whether we will be prepared, whatever transpires. The Pentagon's Yoda knows:

"Difficult to see. Always in motion is future."


Illustration of Thermohaline Circulation

Chart of recent abrupt climate changes

Illustration of abrupt climate change effects

Further Reading

Ocean and Climate Institute links: Abrupt Climate Change

Abrupt Climate Change: Inevitable Surprises by National Academies Press

Pentagon Report links: Environmental Media Services